The invention relates generally to transistors, and, more particularly, to a heterojunction bipolar transistor (IBT) having a thin gallium arsenide antimonide (GaAsSb) base.
Heterojunction bipolar transistors (BBTs) have become state of the art, particularly in npn form, for applications in which high switching speeds and high frequency operation are desired.
An indium gallium arsenide (InGaAs) base HBT typically includes an indium phosphide (InP) or aluminum indium arsenide (AlInAs) emitter that is lattice matched to an InGaAs base and doped with carbon (C) or beryllium (Be) in the range of 3-6xc3x971019 acceptors/cm3. This has led to the development of HBTs having bases of InGaAs at least as thick as 50 nanometers (nm). This minimum thickness has been arrived at due to the generally accepted upper limit of base doping of about 6xc3x971019 acceptors/cm3, and the generally accepted lithography limit of about 1 micron (xcexcm). This leads to the conclusion that the use of a substantially thinner base would lead to an excessively large base resistance, rendering the HBT inadequate for high frequency operation due to the resultant low maximum operational frequency fMAX. The approximate relationship fMAX={square root over (fT/8xcfx80RbCc)}, where fT is the current-gain-bandwidth product, or cutoff frequency, Rb is the base series resistance and Cc is the collector-base capacitance, shows that as the base resistance increases, fMAX is reduced. The relationship is approximate because it is based on a simple lumped-element model of the transistor. Actually, the base series resistance Rb and collector-base capacitance Cc are distributed. More accurate expressions are algebraically complex and would obscure, rather than illuminate, the points that this approximate expression is used to make.
The base series resistance, Rb, can be reduced by scaling down the lithography used to fabricate the transistor, but the practical lower limit is approximately 1 xcexcm. Rb can also be reduced by increasing the base doping, but in ordinary bipolar transistors the emitter efficiency, and hence the current gain, will be seriously compromised as the magnitude of the base doping approaches that of the emitter. This effect of base doping on emitter efficiency is reduced or removed in an HBT due to the wider bandgap of the emitter material compared to the base material. The emitter having a wider bandgap than the base reduces the reverse injection from the base into the emitter even at high base doping. However, in existing HBTs, the base doping cannot be increased without limit because the current gain becomes too small for other reasons.
The doping dependence of the base sheet resistance, transit time, and recombination time is given by the equations:
xcfx81B=1/qNAxcexcpWB≈Ko/NA(1xe2x88x92xcex5)WBxe2x80x83xe2x80x83(Eq. 1) 
1/xcfx84B=1/xcfx84SRH+1/xcfx84RAD+1/xcfx84AUGER=A+BNA+CNA2, andxe2x80x83xe2x80x83(Eq. 2)                               τ          T                =                                            W              B                                      V              thermal                                +                                    W              B              2                                      2              ⁢                              D                n                                                                        (                  Eq          .                      xe2x80x83                    ⁢          3                )            
The term xcfx81B is the base sheet resistance, q is the charge of an electron, NA is the doping concentration in the base, xcexcp is the hole mobility in the base, WB is the base thickness, Ko is a constant, and xcex5 is a constant that can be used to empirically describe the dependence of hole mobility on doping. The term xcfx84B is the net electron lifetime in the base, which has the relationship shown to the lifetimes xcfx84SRH for Shockley-Read-Hall recombination, xcfx84RAD for radiative recombination, and xcfx84AUGER for Auger recombination. A, B, and C are constants that can be use to empirically describe the dependence of xcfx84B on the base doping. The base transit time xcfx84T depends as shown on the base thickness, the electron thermal velocity vthermal, and the electron diffusion constant Dn.
The constants Ko, xcex5, A, B, C, and Dn in equations 1, 2 and 3 are material dependent. Therefore, the scaling behavior with respect to the thickness of the base layer is material dependent. The values for InGaAs lattice-matched to InP are well known, and therefore, will be used as an example when scaling the base of an HBT. For most degenerately doped materials (nearly always the case in HBT bases) xcexcp is weakly dependent on the doping level, and xcex5 is a small positive number. Assuming that this dependence is negligible, and using representative numbers for the other material parameters obtained from R. K. Ahrenkiel et al., xe2x80x9cRecombination lifetime of In0.53 Ga0.47 As as a function of doping density,xe2x80x9d Appl. Phys. Lett., V.72, pp. 3470 ff. (1998); and from Y. Betser and D. Ritter, xe2x80x9cElectron transport in heavily doped bases of InP/GaInAs HBTs probed by magnetotransport experiments,xe2x80x9d IEEE Trans. Elec. Dev., V.43, pp. 1187 ff. (1996), the scaling behavior for an InGaAs base HBT is obtained.
FIG. 1 illustrates the scaling behavior for fT, fMAX, and the base doping in an InGaAs base HBT constrained to have a current gain of 50 as a function of the base layer thickness. The cutoff frequency fT and the maximum operating frequency fMAX are shown with respect to the left axis and the base doping is shown with respect to the right axis. As illustrated, due to the strong dependence of base recombination on doping density, scaling below a base thickness of approximately 36 nm significantly degrades fMAX. This occurs because the base sheet resistance increases as the base thickness is reduced.
Therefore, there is a need in the industry for an HBT having a high current gain that can be maintained at high operating frequencies.
The invention is an HBT having an InP collector, a GaAsSb base and an InP emitter in which the base is constructed using a thin layer of GaAsSb. The thin base layer can be constructed of a GaAsSb material with a composition having a bulk lattice constant that matches the bulk lattice constant of the material of the collector. The thickness of the GaAsSb base layer is less than 49 nm, and preferably less that about 20 nm. A high base doping level is used to reduce the sheet resistivity and lower the base series resistance that results from the thinly grown base layer. The emitter may also be constructed using AlInAs in a composition that results in a lattice-matched with the InP collector.
In an alternative embodiment, the thin base layer is of a GaAsSb composition that includes a higher As content, resulting in a low conduction band energy discontinuity at the emitter-base junction. The thin layer of GaAsSb forming the base of the HBT can be grown using a composition that includes an arsenic fraction of as high as approximately 65%. While providing an advantageous decrease in the conduction band energy discontinuity, an arsenic fraction in the GaAsSb of greater than about 51% changes the lattice parameter of the GaAsSb base layer so that, if grown conventionally thick, undesirable dislocations would occur. To prevent the dislocations from forming, the GaAsSb base layer is grown to a thickness not to exceed the critical thickness at which dislocations would form. Such a GaAsSb base layer has a lattice constant that conforms to the lattice constant of the collector because it is pseudomorphically xe2x80x9cstrainedxe2x80x9d over the collector. A high base doping level is used to reduce the sheet resistivity and lower the base series resistance that results from the thinly grown base layer.